1. Field of the Invention
The invention relates to air purifiers.
2. Description of Prior Art
Air purifiers each comprise a cabinet, incorporating an electrically driven fan and a filter, arranged normally to be free-standing or mounted to a wall of a room to circulating air in the room through the filter. The fan is switched manually ON and OFF, or may be timed to remain ON for a fixed time period before turning OFF automatically. The filter is exchangeable and it is known to arrange a filter replacement indicator, to warn the user that the filter may be dirty, based on a total time the fan has been operating with a particular filter. This may not be a practically efficient way of providing such indication and in any event the present air purifiers provide no indication of air quality in the room.
It is an object of the invention to overcome or at least reduce these problems.
According to the invention there is provided an air purifier comprising a cabinet, a replaceable filter mounted in the cabinet, an electrically driven fan arranged to draw air through the filter, an air quality sensor arranged to detect particles in the air, and a programmed microcomputer arranged to alter the speed of the fan in accordance with signals generated by the air quality sensor in a manner such that the fan speed is increased when the air quality condition is poor or deteriorates, the fan speed is decreased when the air quality condition is good or improves, and the fan is automatically turned off whenever the number of particles per unit volume of air is in the range 0 to 2200. Preferably, a unit volume is one cubic foot.
Preferably, the air quality sensor is arranged to detect particles equal to or larger than one micrometer in size, and the microcomputer is arranged to turn off the fan automatically whenever the number of said particles per cubic foot of air is in the range 0 to 2200.
The air purifier may also included a visual display arranged to provide a visual display indicative of air quality, and the programmed microcomputer is responsive to signals from the air quality sensor at predetermined intervals to cause the visual display to indicate air quality, and to alter the speed of the fan in accordance with the indicated air quality. The period of the predetermined interval is preselectable.
The air purifier may further include a visual display arranged to provide a visual indication when the filter should be replaced, the programmed microcomputer arranged to periodically alter the speed of the fan in accordance with signals generated by the air quality sensor and to cause the visual display to indicate that the filter should be replaced, said indication being caused when a fan total running time value equals or exceeds a predetermined value, the fan total running time value being periodically incremented according to the formula Tn+1=Tn+a X t, wherein Tn is the fan total running time, a is a fan speed factor and t is the fan operating time at speed factor a. The fan total running time value is incremented each time the microcomputer changes the fan speed.